Astronomy & Astrophysics manuscript no. 32853_corr c ESO 2019 March 27, 2019 The visible and near-infrared spectra of asteroids in cometary orbits J. Licandro1; 2, M. Popescu1; 2; 3, J. de León1; 2, D. Morate1; 2, O. Vaduvescu4; 1; 2, M. De Prá5, and V. Alí-Lagoa6 1 Instituto de Astrofísica de Canarias (IAC), CVía Láctea sn, 38205 La Laguna, Spain 2 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 3 Astronomical Institute of the Romanian Academy, 5 Cu¸titulde Argint, 040557 Bucharest, Romania 4 Isaac Newton Group of Telescopes, Apto. 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain 5 Departamento de Astrofísica, Observatório Nacional, Rio de Janeiro, 20921-400, Brazil 6 Max-Planck-Institut fur extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany Received XXX xx, XXX; accepted XXX xx, XXX ABSTRACT Context. Dynamical and albedo properties suggest that asteroids in cometary orbits (ACOs) are dormant or extinct comets. Their study provides new insights for understanding the end-states of comets and the size of the comet population. Aims. We intend to study the visible and near-infrared (NIR) spectral properties of different ACO populations and compare them to the independently determined properties of comets. Methods. We select our ACOs sample based on published dynamical criteria and present our own observational results obtained using the 10.4m Gran Telescopio Canarias (GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory, in Hawaii. We include in the analysis the spectra of ACOs obtained from the literature. We derive the spectral class and the visible and NIR spectral slopes. We also study the presence of hydrated minerals by studying the 0.7 µm band and the UV-drop below 0.5 µm associated with phyllosilicates. Results. We present new observations of 17 ACOs, 11 of them observed in the visible, 2 in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs obtained from the literature. All but two ACOs have a primitive-like class spectrum (X or D-type). Almost 100% of the ACOs in long-period cometary orbits (Damocloids) are D-types. Those in Jupiter family comet orbits (JFC-ACOs) are ∼ 60% D-types and ∼ 40% X-types. The mean spectral slope S 0 of JFC-ACOs is 9.7 ± 4.6 %/1000 Å and for the Damocloids this is 12.2 ± 2.0 %/1000 Å. No evidence of hydration on the surface of ACOs is found from their visible spectra. The spectral slope and spectral class distribution of ACOs is similar to that of comets. Conclusions. The spectral taxonomical classification and the spectral slope distribution of ACOs, and the lack of spectral features indicative of the presence of hydrated minerals on their surface, strongly suggest that ACOs are likely dormant or extinct comets. Key words. minor planets, asteroids, comets, spectroscopy 1. Introduction objects have Halley-type or long period comet (LPC)-type orbits (also known as “Damocloids”, Jewitt 2005). The classical clas- Asteroids in cometary orbits (ACOs) are objects in typical sification of comets, depending on whether their orbital periods cometary orbits that have never shown any kind of activity. Sev- are less than or greater than 200 yr, provides a good discriminant eral classification schemes have been used to identify ACOs, all between comets originated in the trans-Neptunian belt (JFCs) or of them based on Tisserand’s parameter (TJ), a constant of mo- the Oort Cloud (LPCs). Therefore, JFC-ACOs and Damocloids tion in the restricted three-body problem related to an object’s should also be scattered objects from the trans-Neptunian belt encounter velocity with Jupiter. The orbits of the large majority and the Oort Cloud, respectively. of asteroids present T > 3, while most comets have orbits with J Observations of other physical properties (albedo, spec- TJ < 3. Fernández et al. (2005) introduced a second criterion using the minimum orbital intersection distance (MOID) with tra, rotation period, etc.) of the ACOs in the Tancredi’s list arXiv:1903.10880v1 [astro-ph.EP] 26 Mar 2019 Jupiter. To avoid objects with stable dynamical evolution incom- are needed to determine whether these objects truly are ex- patible with the chaotic dynamics of comets, Tancredi (2014) tinct/dormant comets or simply asteroids that have escaped from presented a criterion to identify ACOs that ensures the selection the main belt into cometary-like chaotic orbits. Recently, Lican- of objects with a dynamical evolution similar to the population dro et al. (2016) determined the geometric albedo (pV ), beam- of periodic comets. Tancredi also produced a list of 331 ACOs ing parameter (η) and size distribution of a significant number based on the proposed classification criterion applied to a sam- of JFC-ACOs and Damocloids from Tancredi’s list using data ple of >500,000 known asteroids that, from a dynamical point from NASA’s Wide-field Infrared Explorer (WISE), and con- of view, are the best known extinct/dormant comet candidates. cluded that the pV - and η-value distributions of ACOs are very ACOs in Tancredi’s list are classified in subclasses similar to the similar to those of JFCs nuclei. cometary classification: 203 objects belong to the Jupiter family Spectral properties of ACOs have also been explored, includ- group (JFC-ACOs); 72 objects are classified as Centaurs; and 56 ing spectrophotometric and spectroscopic studies in the visible Article number, page 1 of 11 A&A proofs: manuscript no. 32853_corr and near-infrared (NIR), and compared to the spectral proper- Marconi 2048 x 4096 pixels CCD detectors, a total unvignetted ties of cometary nuclei (Licandro et al. 2006, 2008; DeMeo & field of view of 7.8 x 7.8 arcmin, and a plate scale of 0.127 "/pix. Binzel 2008; Alvarez-Candal 2013; Jewitt 2005, 2015). These To increase the signal-to-noise ratio (S/N) we selected the 2 x 2 studies feature a significant number of asteroids that do not sat- binning and the standard operation mode with a readout speed isfy Tancredi’s criterion, and therefore include several objects of 200 kHz (with a gain of 0.95 e-/ADU and a readout noise of that are unlikely comets. Visible and NIR spectral properties of 4.5 e-). The tracking of the telescope was at the asteroid proper 39 ACOs are presented in Licandro et al. (2008), but only 6 of motion. them are in Tancredi’s list. Another 6 objects from the 55 ACOs GTC spectra were obtained in service mode using the R300R studied by DeMeo & Binzel (2008) are included in this list. On grism in combination with a second-order spectral filter that pro- the other hand, the large majority of Damocloids in Jewitt (2005, duces a spectrum in the 4800 to 9000 Å with a dispersion of 7.74 2015) are in Tancredi’s list. Therefore, the sample of ACOs in Å/pix for a 0.6" slit width. We used a 5.0" width slit oriented in Tancredi (2014), in particular the JFC-ACOs, with known spec- parallactic angle to account for possible variable seeing condi- tral properties, is small (∼ 4% of the sample). This sample needs tions and minimize losses due to atmospheric dispersion. Series to be enlarged. of three spectra were taken for all the targets. Consecutive spec- The aim of this paper is to study the visible and NIR spectral tra were shifted in the slit direction by 10" to correct for fringing. properties of ACOs selected using Tancredi’s criteria, compare Observational details are listed in Table 2. them with the spectral properties of comets and validate Tan- credi’s hypothesis: that these are the best dormant comet candi- dates. We present new spectra of 17 ACOs, 11 of them observed 2.1.2. 4.2m WHT observations in the visible, 2 in the NIR and 4 in the visible and NIR, and Low-resolution visible spectroscopy of four ACOs were ob- analyze the spectral properties of the ACO population using also tained using the double armed Intermediate dispersion Spec- the spectra of 12 ACOs obtained from the literature. trograph and Imaging System (ISIS) at the 4.2m WHT. The The paper is organized as follows. In Sect. 2, observations, R300B grating centered at 4.600Å was used in the BLUE arm data reduction, and spectral extraction are described. In Sect. 3, of ISIS covering the 0.38 − 0.54 µm region with a dispersion the spectra of ACOs retrieved from the literature are presented. In Sect. 4, we analyze the spectral properties of the ACO pop- of 0.86 Å/pixel for a 1" slit width. The R158R grating centered ulation (taxonomical and spectral slope distribution, and hydra- at 7.500Å was used in the RED arm of ISIS covering the 0.50 tion signatures) using the spectra presented in the preceding sec- − 0.95 µm region with a dispersion of 1.63 Å/pixel. A 2" slit tions. We present the results and analysis considering two ACO width was used oriented in the parallactic angle. The tracking of populations: those in Jupiter family comet orbits (JFC-ACOs) the telescope was at the asteroid proper motion. Three spectra and those in Halley-type orbits (Damocloids), assuming they of each object were obtained by shifting the object 5" in the slit likely have an origin in two different comet reservoirs: the trans- direction to better correct the fringing in the RED arm.